Method for producing granular metallic iron

ABSTRACT

A method for producing granular metallic iron of the present invention includes: an agglomeration step of obtaining agglomerates through agglomeration of a mixture that contains an iron oxide-containing material and a carbonaceous reducing agent; and a granulation step of obtaining granular metallic iron by heating the agglomerates, reducing iron oxides in the agglomerates, aggregating generated metallic iron to be granular while separating the metallic iron from slag generated as a by-product, and thereafter cooling and solidifying the metallic iron, wherein agglomerates satisfying all the conditions given by formulas (1) to (3) below are used as the agglomerates: (1) [(total CaO amount+total SiO 2  amount+total Al 2 O 3  amount)/total Fe amount]≧0.250; (2) (total CaO amount/total SiO 2  amount)≧0.9; (3) [total Al 2 O 3  amount/(total CaO amount+total SiO 2  amount+total Al 2 O 3  amount)]×100≧9.7. In the formulas, the total CaO amount, the total SiO 2  amount, the total Al 2 O 3  amount and the total Fe amount respectively represent the mass percentage of CaO, the mass percentage of SiO 2 , the mass percentage of Al 2 O 3  and the mass percentage of Fe contained in the agglomerates.

TECHNICAL FIELD

The present invention relates to a method for producing granular metallic iron. More particularly, the present invention relates to a method for producing granular metallic iron by heating agglomerates that contain an iron oxide-containing material such as iron ore and a carbonaceous reducing agent such as a carbonaceous material.

BACKGROUND ART

A direct iron-making method has received attention as a method for producing granular metallic iron through reduction of iron oxides present in iron oxide-containing materials such as iron ore. The direct iron-making method is a process in which granular metallic iron is produced using coal, relatively easy to procure, as a carbonaceous reducing agent. The process involves charging, into a heating furnace, agglomerates that contain an iron oxide-containing material and a carbonaceous reducing agent, heating the agglomerates inside the furnace through gas heating by heating burners and/or using radiant heat, reducing iron oxides in the agglomerates by means of this heating, aggregating generated metallic iron to be granular while separating the metallic iron from slag generated as a by-product, and thereafter, cooling and solidifying the metallic iron, to obtain granular metallic iron. The above process is advantageous in that, aside from using coal as the carbonaceous reducing agent, a powdery iron ore can be used as the iron oxide-containing material. As a further advantage, the iron oxide-containing material and the carbonaceous reducing agent stand close to each other during reduction, and hence the iron oxides can be reduced quickly, and the content of carbon in the granular metallic iron that is obtained through reduction can be adjusted easily. The applicant had already proposed the techniques disclosed in Patent Documents 1 to 8, as methods for producing granular metallic iron with the above process.

CITATION LIST Patent Documents

Patent Document 1: Japanese Unexamined Patent Publication No. 2004-285399

Patent Document 2: Japanese Unexamined Patent Publication No. 2009-7619

Patent Document 3: Japanese Unexamined Patent Publication No. 2009-270193

Patent Document 4: Japanese Unexamined Patent Publication No. 2009-270198

Patent Document 5: Japanese Unexamined Patent Publication No. 2010-189762

Patent Document 6: Japanese Unexamined Patent Publication No. 2013-142167

Patent Document 7: Japanese Unexamined Patent Publication No. 2013-174001

Patent Document 8: Japanese Unexamined Patent Publication No. 2013-36058

SUMMARY OF INVENTION

As described above, when the iron oxides are reduced through heating of the agglomerates, the metallic iron is generated while the slag is also generated as a by-product. When the amount of the slag generated as a by-product is large, aggregation of the metallic iron is inhibited, and the granular metallic iron becomes fine-grained and difficult to separate from the slag. The yield of the granular metallic iron drops as the result. In the production of granular metallic iron in accordance with the methods described in the Patent Documents 1 to 8, the amount of slag generated as a by-product is small, and such a problem does not arise. Meanwhile, methods for producing granular metallic iron where the amount of slag generated as a by-product is large are required not to exhibit a drop in the yield of the granular metallic iron.

To cut production costs in the above process, it is desirable to increase the yield of the granular metallic iron, and to produce the metallic iron quickly and with good productivity. A conceivable way of increasing the yield of the granular metallic iron is to increase the ratio of the granular metallic iron having little slag mixed thereinto (for instance, granular metallic iron remaining on a sieve having 3.35 mm openings), from among the granular metallic iron that is obtained. Meanwhile, for instance increasing the amount of the carbonaceous reducing agent in the agglomerates is a conceivable approach in terms of producing metallic iron quickly. That is because carburizing and melting of metallic iron at the outer periphery of the agglomerates are promoted, and the metallic iron is generated quickly, as a result of increasing the mixing amount of the carbonaceous reducing agent. When the mixing amount of the carbonaceous reducing agent is increased, however, there arises an excess of carbonaceous reducing agent at the center of the agglomerates, aggregation of metallic iron is inhibited, and the granular metallic iron becomes finer. The yield drops and productivity is poor when the granular metallic iron is fine-grained.

It is an object of the present invention to provide a method for producing granular metallic iron that allows increasing productivity of granular metallic iron, as compared with conventional art, in cases of significant amount of slag formed as a by-product during generation of metallic iron.

One aspect of the present invention is directed to a method for producing granular metallic iron, including: an agglomeration step of obtaining agglomerates through agglomeration of a mixture that contains an iron oxide-containing material and a carbonaceous reducing agent; and a granulation step of obtaining granular metallic iron by heating the agglomerates, reducing iron oxides in the agglomerates, aggregating generated metallic iron to be granular while separating the metallic iron from slag generated as a by-product, and thereafter cooling and solidifying the metallic iron; wherein agglomerates satisfying all the conditions given by formulas (1) to (3) are used as the agglomerates.

[(Total CaO amount+total SiO₂ amount+total Al₂O₃ amount)/total Fe amount]≧0.250   (1)

(Total CaO amount/total SiO₂ amount)≧0.9   (2)

[Total Al₂O₃ amount/(total CaO amount+total SiO₂ amount+total Al₂O₃ amount)]×100≧9.7   (3)

In the formulas (1) to (3), the total CaO amount, the total SiO₂ amount, the total Al₂O₃ amount and the total Fe amount respectively represent the mass percentage of CaO, the mass percentage of SiO₂, the mass percentage of Al₂O₃ and the mass percentage of Fe contained in the agglomerates.

The objects, features, aspects and advantages of the present invention will become clearner through reference to the following detailed description and drawings.

DESCRIPTION OF EMBODIMENTS

The inventors conducted extensive research directed at increasing the productivity of granular metallic iron in cases where the amount of slag generated as a by-product is large. As a result, the inventors found that the productivity of granular metallic iron is increased if the mass percentage of CaO, the mass percentage of SiO₂, the mass percentage of Al₂O₃ and the mass percentage of Fe (also referred to in the present specification as total CaO amount, total SiO₂ amount, total Al₂O₃ amount and total Fe amount, respectively) contained in the agglomerates satisfy predetermined relationships, and perfected the present invention on the basis of that finding.

Conventional methods for producing granular metallic iron were problematic in that aggregation of metallic iron was inhibited, and the yield of granular metallic iron dropped, accompanying increase in the amount of slag generated as a by-product during reduction of iron oxides in the agglomerates. In particular, the drop in yield of granular metallic iron was significant when basicity (total CaO amount/total SiO₂ amount) calculated on the basis of the total CaO amount and the total SiO₂ amount was 0.9 or higher. Therefore, the inventors addressed the causes of drops in yield and found that dicalcium silicate (2CaO.SiO₂) was generated as a result of a reaction between CaO and SiO₂ contained in the agglomerates during the heating process (i.e. halfway during the rise in the temperature of the agglomerates) in the method for producing granular metallic iron. Dicalcium silicate has a high melting point, of 2130° C., and has poor reactivity with FeO_(x) It was found that, as a result, generation of dicalcium silicate made the composition of the slag heterogeneous as a whole, which in turn resulted in delayed slag melting. A delay in slag melting inhibited aggregation of metallic iron obtained through reduction of iron oxides contained in the agglomerates. As a result, the granular metallic iron became fine-grained, the yield of granular metallic iron dropped, and productivity could not be increased.

Accordingly, the inventors conducted further research directed at increasing the productivity of granular metallic iron, and found that generation of dicalcium silicate during heating of the agglomerates was suppressed and that a molten phase of CaO—SiO₂—Al₂O₃ system was generated, when the ratio of the total Al₂O₃ amount in the agglomerates with respect to the amount of slag, being a summation of the total CaO amount, the total SiO₂ amount and the total Al₂O₃ amount, was set to be equal to or higher than a predetermined ratio. Homogenization of the composition of the slag as a whole proceeded quickly, and melting progressed rapidly, when generation of dicalcium silicate was suppressed. The heating time of the agglomerates was able to be shortened when the slag melted quickly. Further, metallic iron aggregated quickly with other metallic iron and then coarsened. The yield of the granular metallic iron was enhanced and productivity increased as a result. The present invention was perfected on the basis of these findings.

The present invention will be explained next in detail.

The method for producing granular metallic iron according to the present invention is based on the premise that the total CaO amount, the total SiO₂ amount, the total Al₂O₃ amount and the total Fc amount satisfy the formula (1) and the formula (2). The word “total” means a summation of one component's amounts in all the starting materials contained in the agglomerates. For instance, the total Al₂O₃ amount represents a mass percentage obtained by calculating each mass of Al₂O₃ in each agglomerate starting material such as iron oxide-containing material and carbonaceous reducing agent, and dividing then the value of the summation (i.e. total mass of Al₂O₃) by the mass of the agglomerates.

[Formula (1)]

In the formula (1), the summation of the total CaO amount, the total SiO₂ amount and the total Al₂O₃ amount represents the amount of slag. The left side of the formula (1) represents the ratio (mass ratio) of the amount of slag to the total Fe amount in the agglomerates. The formula (1) being satisfied indicates that the amount of slag in the agglomerates is large with respect to the total Fe amount in the agglomerates. In the present specification, an instance where the amount of slag formed as a by-product during generation of metallic iron is large corresponds to an instance where the formula (1) is satisfied. Thus the effect of enhancing the productivity of granular metallic iron through suppression of generation of dicalcium silicate is elicited by virtue of the fact that the total Al₂O₃ amount and the amount of slag in the agglomerates satisfy the formula (3) in a state where the slag is present in the agglomerates in a significant amount. In the present invention the value on the left side of the formula (1) is set to 0.250 or greater, preferably 0.280 or greater, and more preferably 0.300 or greater. The upper limit of the value on the left side of the formula (1) is not particularly limited, but when the value exceeds 1.00 and the amount of slag is thus excessive, the aggregation of metallic iron is significantly inhibited, and the productivity of granular metallic iron cannot be increased. Therefore, the upper limit of the value on the left side of the formula (1) is preferably for instance 1.00 or smaller, more preferably 0.90 or smaller, yet more preferably 0.80 or smaller, and particularly preferably 0.70 or smaller.

[Formula (2)]

The left side of the formula (2) represents basicity (mass ratio represented by total CaO amount/total SiO₂ amount) calculated on the basis of the total CaO amount and the total SiO₂ amount in the agglomerates. The above dicalcium silicate is generated readily, particularly when basicity is 0.9 or higher. Accordingly, the effect of enhancing the productivity of granular metallic iron through suppression of generation of dicalcium silicate, in agglomerates of such high basicity, is elicited by virtue of the fact that the total Al₂O₃ amount in the agglomerates and the amount of slag in the agglomerates satisfy the formula (3). The lower limit value of the basicity is preferably 1.0 or higher, more preferably 1.1 or higher. The upper limit of the basicity is not particularly limited, but is preferably for instance 2.0 or lower, more preferably 1.8 or lower and yet more preferably 1.5 or lower.

[Formula (3)]

In the present invention it is important that the formula (1) and the formula (2) be satisfied by the total CaO amount, the total SiO₂ amount, the total Al₂O₃ amount and the total Fe amount, and, in addition, that the formula (3) be satisfied by the total Al₂O₃ amount, and the amount of slag resulting from summating the total CaO amount, the total SiO₂ amount and the total Al₂O₃ amount. The value on the left side of the formula (3) (mass percentage of the total Al₂O₃ amount with respect to the amount of slag in the agglomerates) may also be referred to hereafter as Z-value.

When the Z-value is smaller than 9.7, the Al₂O₃ amount is insufficient, and accordingly generation of dicalcium silicate cannot be suppressed. As the result, the productivity of the granular metallic iron cannot be enhanced. In the present invention, therefore, the Z-value is preferably set to 9.7 mass % or higher, preferably 10.0 mass % or higher and yet more preferably 11.0 mass % or higher. The upper limit of the Z-value is not particularly limited, but when the total Al₂O₃ amount in the agglomerates is excessive, there rises the melting point of the slag at the time of finishing slag homogenization, the slag does not melt, and aggregation of metallic iron is inhibited. As the result, the productivity of the granular metallic iron may fail to be raised. Therefore, the Z-value is for instance preferably 60.0 mass % or lower, more preferably 50.0 mass % or lower, and yet more preferably 40.0 mass % or lower.

The method for controlling the values on the left side in the formulas (1) to (3) to lie within the predetermined range is not particularly limited, and for instance it suffices to adjust the mixing amount of the starting materials, such as iron oxide-containing material and carbonaceous reducing agent, that are incorporated into the agglomerates.

The value on the left side of the formula (3) may be adjusted by adding an Al₂O₃-containing material to the above mixture.

As the Al₂O₃-containing material, there can be used for instance aluminum hydroxide, Al₂O₃ powder, bauxite, boehmite, gibbsite, diaspore, kaolinite, kaolin, mullite or the like. For instance the above aluminum hydroxide can be procured from Kojundo Chemical Laboratory Co., Ltd.

The total CaO amount, the total SiO₂ amount and the total Al₂O₃ amount can be quantified by resorting to a known means, for instance ICP (inductively coupled plasma) emission spectroscopy or the like.

The total Fe amount can be quantified by resorting to a known means, for instance potassium dichromate titration or the like.

The method for producing granular metallic iron according to the present invention will be explained next.

The method for producing granular metallic iron according to the present invention involves heating agglomerates obtained through agglomeration of a mixture that contains an iron oxide-containing material and a carbonaceous reducing agent, reducing iron oxides in the agglomerates, aggregating generated metallic iron to be granular while separating the metallic iron from slag generated as a by-product, and then cooling and solidifying, to produce thereby granular metallic iron. As described above, agglomerates that satisfy all the conditions given by the formulas (1) to (3) are used herein as the above agglomerates.

An explanation follows next on a step of obtaining agglomerates through agglomeration of a mixture that contains an iron oxide-containing material and a carbonaceous reducing agent (also referred to as agglomeration step in the present specification), and a step of heating the obtained agglomerates, reducing iron oxides in the agglomerates, aggregating generated metallic iron to be granular while separating the metallic iron from slag generated as a by-product, and thereafter cooling and solidifying the metallic iron, to obtain granular metallic iron (also referred to as granulation step in the present specification).

Agglomeration Step

In the agglomeration step, agglomerates are obtained through agglomeration of a mixture that contains an iron oxide-containing material and a carbonaceous reducing agent.

As the iron oxide-containing material, there can be used for instance iron ore, iron sand, ironmaking dust, nonferrous smelting residue, ironmaking waste or the like. As the iron ore, there can be used for instance hematite ore from Australia or India. The hematite ore from these producing areas has a higher Al₂O₃ content than other hematite ore from other areas.

For instance coal, coke or the like can be used as the carbonaceous reducing agent.

It suffices herein that the carbonaceous reducing agent has a carbon content that allows reducing iron oxides in the iron oxide-containing material. Specifically, the content may lie in a range of 0 to 5 mass % surplus or 0 to 5 mass % shortfall with respect to the carbon amount that allows reducing iron oxide present in the iron oxide-containing material. That is, the carbon content may be within ±5 mass % with respect to the carbon amount that allows reducing iron oxides contained in the iron oxide-containing material.

At least one type selected from the group consisting of melting point adjusting agents and binders may be further added to the mixture that contains the iron oxide-containing material and the carbonaceous reducing agent.

The above melting point adjusting agent represents a substance having the effect of lowering the melting point of gangue in the iron oxide-containing material and the melting point of ash in the carbonaceous reducing agent. Specifically, addition of a melting point adjusting agent to the above mixture makes it possible to influence the melting point of components other than iron oxides contained in the agglomerates, such as gangue or the like, for instance in that the melting point can be lowered as the result. Melting of gangue and forming of molten slag are promoted as the result. In this period parts of iron oxides dissolve in the molten slag, and then are reduced in the molten slag to be metallic iron. The metallic iron generated in the molten slag comes in contact with reduced metallic iron having undergone reduction while solid, to elicit thereby aggregation in the form of solid metallic iron.

For instance a CaO supplying material, a MgO supplying material, a SiO₂ supplying material or the like can be used as the above melting point adjusting agent. As the CaO supplying material, there can be used at least one type selected from the group consisting of for instance CaO (quicklime), Ca(OH)₂ (slaked lime), CaCO₃ (limestone) and CaMg(CO₃)₂ (dolomite). As the MgO supplying material, there can be used at least one type selected from the group consisting of MgO powder, Mg-containing materials extracted for instance from natural ores, seawater or the like, and MgCO₃. For instance SiO₂ powder, silica sand or the like can be used as the SiO₂ supplying material.

A polysaccharide such as a starch, for instance cornstarch, wheat flour or the like, can be used as the binder.

Preferably, the iron oxide-containing material, the carbonaceous reducing agent and the melting point adjusting agent are pulverized beforehand prior to being mixed. Preferably, for instance, the iron oxide-containing material is pulverized beforehand to an average particle size of 10 to 60 μm, the carbonaceous reducing agent to an average particle size of 10 to 1000 μm, and the melting point adjusting agent to an average particle size of 5 to 90 μm.

The means for pulverizing the iron oxide-containing material and so forth is not particularly limited, and known means can be resorted to herein. For instance, pulverizing can be accomplished using a vibration mill, a roll crusher, a ball mill or the like.

A mixer of rotating vessel type or fixed vessel type can be used to mix the starting materials of the agglomerates. The mixer of rotary vessel type is not particularly limited, and for instance a rotary cylindrical mixer, a double conical mixer, a V-type mixer or the like can be used herein. The mixer of fixed vessel type is not particularly limited, and for instance there can be used a mixer provided with rotating vanes, such as plows, in a mixing tank.

As an agglomerating machine for agglomeration of the above mixture there can be used for instance a pan granulator, a cylindrical granulator, a twin roll-type briquetting machine or the like.

The shape of the agglomerates is not particularly limited, and the agglomerates may be shaped by pelletization, briquetting or extrusion.

Granulation Step

In the granulation step, granular metallic iron is obtained by heating the agglomerates obtained in the agglomeration step, reducing iron oxides in the agglomerates, aggregating generated metallic iron to be gr+anular while separating the metallic iron from slag generated as a by-product, and thereafter cooling and solidifying the metallic iron.

Heating of the agglomerates can be accomplished for instance in an electric furnace or in a moving hearth-type heating furnace.

The above moving hearth-type heating furnace is a heating furnace in which the hearth can move within the furnace in the manner of a belt conveyor. Examples thereof include for instance a rotary hearth furnace and a tunnel furnace.

The above rotary hearth furnace is provided with a hearth that rotates about the center of the furnace and the hearth is circular or doughnut -shaped such that the starting point and the end point of it are the same position. The iron oxides contained in the agglomerates that are charged into the hearth of the rotary hearth furnace are thermally reduced while going round in the interior of the furnace, whereupon metallic iron is generated as a result. Accordingly, a charging means for charging the agglomerates is provided, in the rotary hearth furnace, at the most upstream side in the rotation direction, and a discharge means is provided at the most downstream side in the rotation direction. The discharge means is actually positioned directly upstream of the charge means, since the rotary hearth furnace has a structure with a rotating hearth. The above the tunnel furnace is a heating furnace in which the hearth moves through the interior of the furnace in a straight direction.

The agglomerates are preferably heated at 1300 to 1500° C. When the heating temperature is lower than 1300° C., metallic iron and slag do not melt readily, and it is difficult to achieve high productivity. When the heating temperature exceeds 1500° C., on the other hand, the temperature of exhaust gas increases, and equipment costs increase on account of the accompanying larger scale of the equipment for exhaust gas treatment.

Preferably, a bed material such as a carbonaceous material or a refractory ceramic is laid on the top face of the hearth, in order to protect the hearth, before charging of the agglomerates into the above electric furnace or moving hearth-type heating furnace.

Aside from the carbonaceous reducing agents exemplified above, refractory particles as well can be used as the bed material.

The particle size of the bed material is preferably for instance 3 mm or smaller, so as to preclude sinking of the agglomerates or molten products thereof. The lower limit of the particle size is preferably for instance 0.5 mm or greater, to prevent the bed material from being blown away by combustion gases from burners.

Others

The granular metallic iron obtained in the granulation step is discharged from the furnace together with, for instance, by-product slag, the bed material that was laid as needed, and so forth, and can be recovered thereafter through sorting by means of a sieve, a magnetic separator or the like.

As explained in detail above, one aspect of the present invention is directed to a method for producing granular metallic iron, including: an agglomeration step of obtaining agglomerates through agglomeration of a mixture that contains an iron oxide-containing material and a carbonaceous reducing agent; and a granulation step of obtaining granular metallic iron by heating the agglomerates, reducing iron oxides in the agglomerates, aggregating generated metallic iron to be granular while separating the metallic iron from slag generated as a by-product, and thereafter cooling and solidifying the metallic iron, wherein agglomerates satisfying all the conditions given by the formulas (1) to (3) are used as the agglomerates.

In the present invention, the components of the agglomerates are adjusted so that the total CaO amount, total SiO₂ amount, total Al₂O₃ amount and total Fe amount in the agglomerates satisfy all the conditions given by the formulas (1) to (3). As the result there is suppressed generation of dicalcium silicate, which has a high melting point and poor reactivity with FeO_(x), in cases of significant amount of slag generated as a by-product in the production of granular metallic iron through heating of the agglomerates. Aggregation of metallic iron generated through reduction of iron oxides is promoted as the result, and the productivity of granular metallic iron is increased as compared with conventional productivity.

In the method for producing granular metallic iron, an Al₂O₃-containing material is preferably added to the mixture, before the agglomeration of the mixture, in the agglomeration step. This allows preparing more accurately and more easily agglomerates that satisfy all the conditions given by the formulas (1) to (3), as the result of which the productivity of granular metallic iron can be increased more accurately and more easily.

Examples

The present invention will be explained next more specifically by way of working examples, but the invention is not limited by these examples, and needless to say can be carried out while including additional modifications within the scope conforming to the gist disclosed heretofore and hereinafter, all such modifications being encompassed within the technical scope of the invention.

Firstly, agglomerates were obtained through agglomeration of a mixture that contained an iron oxide-containing material and a carbonaceous reducing agent, and that was further blended thereinto a melting point adjusting agent, an Al₂O₃-containing material and a binder, as additives. The specific procedure was as follows.

Iron ore A having the component composition shown in Table 1 was used as the iron oxide-containing material. Table 1 also shows amount of slag, which is the mass percentage of slag obtained by summating a CaO amount being the mass percentage of CaO, a SiO₂ amount being the mass percentage of SiO₂, and an Al₂O₃ amount being the mass percentage of Al₂O₃ in the iron ore A

Coal A having the component composition shown in Table 2 was used as the carbonaceous reducing agent. Table 2 also shows an Al₂O₃ amount being the mass percentage of Al₂O₃ in the coal A, as well as amount of slag, which is the mass percentage of slag obtained by summating a CaO amount being the mass percentage of CaO, a SiO₂ amount being the mass percentage of SiO₂, and an Al₂O₃ amount being the mass percentage of Al₂O₃ in the coal A.

Limestone having the component composition shown in Table 3 was used as the melting point adjusting agent. Table 3 also shows amount of slag, which is the mass percentage of slag obtained by summating a CaO amount being the mass percentage of CaO, a SiO₂ amount being the mass percentage of SiO₂, and an Al₂O₃ amount being the mass percentage of Al₂O₃, in the limestone.

Aluminum hydroxide reagent produced by Kojundo Chemical Laboratory Co., Ltd. was used as the A1₂0₃-containing material. The component composition of this reagent is shown in Table 3. Table 3 also shows amount of slag, which is the mass percentage of slag obtained by summating a CaO amount being the mass percentage of CaO, a SiO₂ amount being the mass percentage of SiO₂, and an Al₂O₃ amount being the mass percentage of Al₂O₃, in the aluminum hydroxide reagent.

Wheat flour was used as the binder.

The iron ore, the coal, the limestone, the aluminum hydroxide reagent and the binder were mixed in the proportions shown in Table 4, with addition of a suitable amount of water; the resulting mixture was pelletized using a tire type pelletizer to obtain green pellets having an average diameter of 19 mm. The obtained green pellets were charged into a dryer and were dried through heating at 180° C. for 1 hour, to remove adhered water. Dry pellets A and B as the agglomerates were obtained as the result.

Table 4 also shows a total Al₂O₃ amount being the mass percentage of Al₂O₃ in the dry pellets A and B, as well as amount of slag, which is the mass percentage of slag obtained by summating a total CaO amount being the mass percentage of CaO, a total SiO₂ amount being the mass percentage of SiO₂, and a total Al₂O₃ amount being the mass percentage of Al₂O₃, in the dry pellets A and B. A total Fe amount, being the mass percentage of Fe in the dry pellets A and B was measured by potassium dichromate titration. The measured values are also shown in Table 4.

The total mass (g) of each 30 pellets of the dry pellets A and B was measured. The measured values are also shown in Table 4.

The value on the left side of the formula (1) was calculated on the basis of the amount of slag in the dry pellets A and B and the total Fe amount in the dry pellets A and B shown in Table 4. The calculated values are shown in Table 5.

The value on the left side of the formula (2) was calculated on the basis of: the component composition of the iron ore, coal, limestone and aluminum hydroxide reagent shown in Tables 1 to 3; and the total CaO amount and total SiO₂ amount calculated on the basis of the compounding ratios shown in Table 4. The calculated values are shown in Table 5.

The Z-value, being the value on the left side of the formula (3) was calculated on the basis of the total Al₂O₃ amount in the dry pellets A and B and the amount of slag in the dry pellets A and B, shown in Table 4. The calculated values are shown in Table 5.

Secondly, each granular metallic iron was obtained by charging the dry pellets A and B respectively into a heating furnace, heating the dry pellets at 1450° C., reducing iron oxides in the dry pellets, aggregating generated metallic iron to be granular while separating the metallic iron from slag generated as a by-product, and thereafter cooling and solidifying the metallic iron. A carbonaceous material (anthracite) having a maximum particle size of 2 mm or smaller was laid on the hearth of the heating furnace prior to charging of the dry pellets A and B respectively, for the purpose of protecting the hearth.

During heating, nitrogen gas was caused to flow within the heating furnace at a flow rate of 30 Nl/min, to form a nitrogen gas atmosphere inside the heating furnace. It is found that the values of productivity ratio described below do not vary even with changes in the components and in flow rate of the gas that flows during heating.

After supply of the dry pellets to the heating furnace, the interior of the heating furnace was visually observed to check whether or not the outermost layer of the dry pellets had melted and iron having separated from slag had taken on a droplet shape. There was also measured the time (seconds) elapsed since supply of the dry pellets into the heating furnace until melting of the outermost layer of the dry pellets and forming droplet-like iron separated from slag. This time was taken as a slag separation completion time (seconds). In the present embodiment, the interior of the heating furnace was visually observed to determine whether or not the outermost surface of the dry pellets had melted and iron having separated from slag had taken on a droplet shape. However, this determination may be carried out by extracting part of the dry pellets from the heating furnace.

Next, the heated product discharged out of the heating furnace was magnetically separated, and then was sifted with a sieve having 3.35 mm openings, thereafter the residue on the sieve was recovered. The over-sieve residue was mainly granular metallic iron that was able to be used as a product (iron source).

The ratio of over-sieve residue with respect to the total mass of iron that is charged into the heating furnace is defined herein as the yield, which was calculated as follows.

Yield (%)=(mass of over-sieve residue/total mass of iron charged into heating furnace)×100

The productivity ratio of No. 2 was calculated in accordance with the following procedure taking the productivity in No. 1 as a reference (1.00). The productivity ratio of No. 2 was defined as the ratio of productivity in No. 2 with respect to productivity in No. 1. The productivity ratio of No. 2 can be calculated for instance in accordance with the following formula.

Productivity ratio of No. 2=productivity in No. 2 (g/sec)/productivity (g/sec) in No. 1

The above productivity can be calculated on the basis of the following formula.

Productivity (g/sec)=[iron amount (g) in over-sieve residue/slag separation completion time (seconds)]

The iron amount contained in the over-sieve residue can be calculated in accordance with the following formula.

Iron amount (g) in over-sieve residue=[total mass (g) of 30 dry pellets×total Fe amount (mass %) in dry pellets/100×yield (mass %) of granular metallic iron/100]

Acceptability non-acceptability or failure was evaluated on the basis of whether all the conditions given by the formulas (1) to (3) were satisfied or not. Each evaluation result of the dry pellets A and B is also shown in Table 5.

Herein No. 1 is a comparative example where the evaluation result is non-acceptable (specifically, the condition given by the formula (3) is not satisfied). By contrast, No. 2 is an example of the present invention in which the evaluation result is acceptable (i.e. all the conditions given by the formulas (1) to (3) are satisfied). The productivity in No. 2 was 5% or more higher than that in No. 1, since No. 2 satisfies all the conditions given by the formulas (1) to (3).

TABLE 1 Component composition (mass %) Slag amount Iron ore T. Fe FeO Al₂O₃ SiO₂ CaO MgO (mass %) A 61.38 1.73 0.89 7.29 0.14 0.08 8.32

TABLE 2 Analysis value (mass %) Component composition of ash Al₂O₃ Slag Volatile (mass %) amount amount Coal T. C Fix. C content Ash SiO₂ CaO Al₂O₃ MgO (mass %) (mass %) A 87.59 77.51 15.77 6.72 44.58 4.86 25.21 1.76 1.69 5.02

TABLE 3 Component composition (mass %) Slag amount Additive CaO MgO SiO₂ Al₂O₃ (mass %) Limestone 56.13 0.15 0.50 0.14 56.77 Aluminum hydroxide 0 0 0 65.38 65.38

TABLE 4 In pellets Mixing ratio (mass %) Total Al₂O₃ Slag Total Fe Total mass of Iron Coal Aluminum amount amount amount 30 pellets Pellets ore A A Limestone hydroxide Binder Total (mass %) (mass %) (mass %) (g) A 71.17 15.86 11.78 0.30 0.90 100 1.11 13.60 43.79 235.95 B 70.76 15.64 11.70 1.00 0.90 100 1.56 13.97 43.54 232.71

TABLE 5 Left side of Left side of Left side of Formula (1) Formula (2) Formula (3) Productivity ratio No. Pellets (mass ratio) (mass ratio) (mass %) Evaluation (−) 1 A 0.311 1.2  8.16 Non-acceptable 1.00 2 B 0.321 1.2 11.17 Acceptable 1.08

The present invention has been appropriately and sufficiently explained above by way of embodiments, for the purpose of illustrating the invention. A person skilled in the art should recognize, however, that the embodiments described above can be easily modified and/or improved. Therefore, it is understood that any modified embodiments or improved embodiments that a person skilled in the art can arrive at are encompassed within the scope as claimed in the appended claims, so long as these modifications and improvements do not depart from the scope of the claims.

INDUSTRIAL APPLICABILITY

By using the method for producing granular metallic iron of the present invention it becomes possible to increase the productivity of granular metallic iron compared to conventional productivity, in cases of significant amounts of slag generated as a by-product. 

1. A method for producing granular metallic iron, comprising: an agglomeration step of obtaining agglomerates through agglomeration of a mixture that contains an iron oxide-containing material and a carbonaceous reducing agent; and a granulation step of obtaining granular metallic iron by heating the agglomerates, reducing iron oxides in the agglomerates, aggregating generated metallic iron to be granular while separating the metallic iron from slag generated as a by-product, and thereafter cooling and solidifying the metallic iron; wherein agglomerates satisfying all the conditions given by formulas (1) to (3) below are used as the agglomerates: [(total CaO amount+total SiO₂ amount+total Al₂O₃ amount)/total Fe amount]≧0.250   (1); (total Cat) amount/total SiO₂ amount)≧0.9   (2); [total Al₂O₃ amount/(total CaO amount+total SiO₂ amount+total Al₂O₃ amount)]×100≧9.7   (3), wherein the total CaO amount, the total SiO₂ amount, the total Al₂O₃ amount and the total Fe amount in the formulas (1) to (3) respectively represent the mass percentage of CaO, the mass percentage of SiO₂, the mass percentage of Al₂O₃ and the mass percentage of Fe comprised in the agglomerates.
 2. The method for producing granular metallic iron of claim 1, wherein in the agglomeration step an Al₂O₃-containing material is added to the mixture, before the agglomeration of the mixture. 